1. Technical Field
The present disclosure relates to an active matrix type organic light emitting display, and more particularly, to an organic light emitting display capable of compensating for luminance variations caused by changes in a driving element over time and a method of manufacturing the same.
2. Discussion of the Related Art
An organic light emitting display is a self-emissive device in which an organic light emitting layer emits light by recombination of electrons and holes, and is expected to be a next-generation display device in that it is high in luminance, low in driving voltage, and ultra-thin in thickness.
Each of a number of pixels included the organic emitting display includes an organic light emitting diode (OLED), which is a light emitting element including an anode, a cathode, and an organic emitting layer formed between them, and a pixel circuit for driving the OLED independently. The pixel circuit usually includes a switching thin film transistor (TFT), a storage capacitor, and a driving element (driving TFT). The switching TFT charges the capacitor with a data voltage in response to a scan signal, and adjusts the emission intensity of the OLED by controlling the magnitude of electrical current supplied to the OLED according to the level of the voltage stored in the capacitor. The emission intensity of the OLED proportional to the current supplied from the driving TFT.
The organic light emitting display have differences in characteristics such as the threshold voltage Vth, mobility, etc. of the driving TFT of each pixel due to process deviations or the like. Hence, the amount of driving current for driving the OLED varies, causing luminance variations between the pixels. In general, initial differences in the characteristics of the driving TFT cause spots or inconsistencies of the display (mura) on the screen, and differences in the characteristics of the driving TFT made over time during the driving of the OLED can reduce the lifespan of the display panel or produce afterimages.
U.S. Pat. No. 7,834,825 discloses a data compensation method which measures electrical current in each pixel and compensates input data according to the measurement result. However, this related art patent is based upon the premise that the characteristics of the driving TFT are not changed after shipment, without taking into account luminance variations caused by changes in a driving TFT over time.
FIG. 1 is a schematic diagram showing a related art organic light emitting display. FIG. 2 is a timing diagram showing the timings of when image display gate signals and sensing gate signals are supplied in the organic light emitting display of FIG. 1.
Korean Laid-Open Patent Publication No. 10-2013-0039551 (also published as U.S. Patent Application Publication No. 2013/0093652) discloses a method for compensating for luminance variations caused by changes in a driving element over time. In this related art technology, as shown in FIG. 2 of the present disclosure, one frame (“1 Frame”) is divided into a display period DP and a vertical blank period VB, and the OLED of each pixel emits light during the display period to display an image and the threshold voltage of a driving TFT (DT) is measured during the vertical blank period VB. In the related art technology, however, a first gate driver 2A for generating image display gate signals D11 to Dn2 and a second gate driver 2B for generating sensing gate signals S11 to Sn2 need to be included as shown in FIG. 1 of the present disclosure. Also, a multiplexer circuit 3 for selectively supplying the gate signals D11 to Dn2 and S11 to Sn2 (collectively labeled “sensing”) input from the gate driver 2 to the display panel needs to be further included. Thus, it is difficult to reduce the bezel area of the display panel. In FIG. 1, “1” denotes a data driver, “RL” denotes a reference line required for sensing, “DL” denotes a data line to which a data voltage is applied, “GLa” denotes a first gate line connected to a first switch TFT ST1 included in a pixel P, “GLb” denotes a second gate line connected to a second switch TFT ST2 included in the pixel P, “DT” denotes a driving TFT included in the pixel P, and “Cst” denotes a storage capacitor included in the pixel P. Each of the pixels P receives high-potential power EVDD and low-potential power EVSS from a power generator (not shown).
Moreover, this related art technology has the problem of display image distortion that occurs when display data applied to pixels to be sensed does not have the same value for one frame but is converted into sensing data during the vertical blank period VB.